The present invention relates to a process for producing planographic printing plates, or more particularly, to a method for producing planographic printing plates comprising subjecting light sensitive planographic printing plate members, usually having metal as a support, to a burning-in heat treatment.
Positive type light sensitive planographic printing plates are usually produced by coating an o-quinone diazide type light sensitive material, either alone or in combination with appropriate additives, on a support such as a metal, plastic, or the like. After exposing this light sensitive printing plate to active rays through a transparent positive, the o-quinone diazide light sensitive material decomposes at the exposed areas and becomes alkali soluble. Thus it is easily removed with an aqueous alkali solution, thereby providing a positive image on the support.
Where a support with a surface which is hydrophilic or has been previously treated to render the same hydrophilic is used, the areas removed by an aqueous alkali developing solution expose the hydrophilic layer, which will receive water and repel ink. On the other hand, the areas remaining as an image are oleophilic and accept ink. As negative type light sensitive compositions, diazonium salts, azide compounds, or photopolymerizable compounds are often used. These light sensitive materials are coated, alone or in combination with appropriate additives, on a support to form negative working printing plates.
In this case, where a support with a surface which is hydrophilic or has been previously treated to render the same hydrophilic is used, unexposed areas are removed with a developer, thus uncovering the hydrophilic layer which will receive water, and repel ink. On the other hand, the areas which have hardened upon exposure to light and remain as an image at development are oleophilic and receive ink.
It has been known in the lithographic art to increase press life capabilities of offset plate systems by up to tenfold when said plates are subjected to elevated heating after exposure and development. Cross-linking of the polymeric image areas occurs under heating conditions thus resulting in virtually complete solvent insolubility, increased abrasion resistance, and a vastly increased press life.
This heating which is generally called a burning-in, is described in detail in British Pat. Nos. 1,151,199 and 1,154,749 and U.S. Pat. No. 4,063,507.
The application of a burning-in makes it possible to increase the number of copies obtained with one printing plate several fold compared to the case where no burning-in treatment is used.
A characteristic side effect of the heating step is alteration of the aluminum substrate from a hydrophilic surface to one of an oleophilic type. It is believed that aluminum oxides present at the interface undergo complex rearrangements catalyzed by high temperature environments, thus resulting in a shift of the hydrophilic/hydrophobic balance.
With printing plates produced by conventional platemaking methods, when special printing inks containing many components capable of dissolving the image on the printing plate, such as an ultraviolet ray curable ink, a low temperature drying ink, etc., are used, the image areas of the printing plate are vigorously dissolved by these inks, resulting in a marked reduction in press life as compared to the case in which an ordinary ink is used. However, the application of a burning-in makes it possible to obtain a sufficient number of copies, even using the above special printing inks, because the solvent resistance of the image areas is markedly increased.
However, when the burning-in is applied, the non-image areas of the printing plate which are hydrophilic prior to the burning-in (i.e., areas where the hydrophilic surface of the support is exposed by development) lose their hydrophilicity and tend to receive printing ink, thereby causing contamination at the background of printed matter.
To negate or reverse the hydrophobic effect created by the heating step, various additional processing steps have been introduced during the overall processing procedure by various plate manufacturers. Claims of post-treatment steps as redeveloping, plate cleaning, plate conditioning, etc., have been suggested for the above. Additionally, pre-baking processing procedures have been recommended to avoid aluminum oxide reactivity.
These steps consist of applying surfactants such as mono and didecylphenoxy benzene disulfonates or water-soluble polymers, such as polyvinyl alcohol or gums such as gum arabic or synthetic gums to the developed plate prior to burning-in. Unfortunately, these water-soluble constituents although hydrophilic initially, underto chemical change under elevated temperature themselves which nullify their desired intent.
After the burning-in, therefore, it had been required that a surface smoothening treatment to restore hydrophilicity by cleaning the non-image areas be applied.
This surface smoothening treatment is achieved by eroding the metal surface of the support with an aqueous alkali or acid solution, thereby exposing a fresh, clean surface. The alkali or acid used is selected from those compounds capable of eroding the metal surface in a short period, and fluorides such as hydrofluoric acid, hydrofluorboric acid, hydrofluosilicic acid, and the like are often used. These fluorides, however, are toxic, dangerous substances, and cause many pollution problems.
Moreover, since the application of the surface smoothening treatment erodes the metal surface of the support, and renders the surface easily scratchable and poorly durable to abrasion, it is likely that the non-image areas will lose the ability to accept water, background contamination will occur, and the printing plate will not be durable to further printing.